Affinity-Tuned CAR T Cells Slay Tumor Cells, Spare Normal Cells

It sounds paradoxical, but chimeric antigen receptor (CAR) T cells can be more easy-going and more effective at killing cancer cells. Engineering a lackadaisical but still-deadly CAR T cell requires affinity tuning, which is a matter of reducing a CAR T cell’s sensitivity to target proteins, that is, proteins that stud cancer cells and, to a lesser degree, normal cells. The idea is to produce CAR T cells that are sufficiently insensitive to ignore normal cells, which are relatively target-sparse, and yet sensitive enough to latch onto cancer cells, which are relatively target-rich.

This approach may prove to be especially useful as CAR T-cell therapies are directed against solid tumors. To date, most work with CAR T cells has focused on leukemia and lymphoma. Although CAR T cells cannot distinguish between malignant B cells and normal B cells in these diseases, the side effects are manageable. But that may not be the case with solid tumors. That is, CAR T cells with potency against solid tumors may damage essential tissues.

Hoping to advance the design of CAR T-cell therapies that would work against solid tumors and still be safe for patients, two independent research teams have published studies about affinity tuning. One team is in the laboratory of Carl June, M.D., at the University of Pennsylvania. The other is in the laboratory of Laurence J.N. Cooper, M.D., Ph.D., at The University of Texas MD Anderson Cancer Center.

The June laboratory effort was led by Yangbing Zhao, M.D., Ph.D. “[We] have been working for the past three years to optimize a system to fine-tune the affinity of single chain variable fragments (scFv)—the part of the CAR T cell that recognizes the tumor target,” Dr. Zhao explained. “[These fragments are designed] to discriminate tumors that have high levels of a protein from normal tissues that have low levels of the same protein.

“Unlike the common expectation that lowering the affinity of CAR T cells might also lower their efficacy, we have shown that lowering the affinity in fact does the opposite—lower-affinity CAR T cells displayed more potent reactivity to tumor cells expressing high levels of the target than did higher-affinity CAR T cells.”

Dr. Zhao is the senior author of a paper that appeared September 1 in Cancer Research. The paper—“Affinity-Tuned ErbB2 or EGFR Chimeric Antigen Receptor T Cells Exhibit an Increased Therapeutic Index against Tumors in Mice”—described how a CAR-expressing T-cell panel was generated with target antigen affinities varying over three orders of magnitude.

A similar approach was taken by Dr. Cooper’s group, which also published a paper that appeared September 1 in Cancer Research. This paper—“Tuning Sensitivity of CAR to EGFR Density Limits Recognition of Normal Tissue While Maintaining Potent Antitumor Activity”—sought to generate CAR T cells “capable of distinguishing malignant from normal cells based on the disparate density of EGFR expression by generating two CARs from monoclonal antibodies that differ in affinity.”

The researchers used two monoclonal antibodies, cetuximab, which has higher affinity for EGFR, and nimotuzumab, which has lower affinity for EGFR, and from these, they engineered high-affinity cetux-CAR T cells and low-affinity nimo-CAR T cells.

“T cells with low-affinity nimotuzumab-CAR selectively targeted cells overexpressing EGFR, but exhibited diminished effector function as the density of EGFR decreased,” wrote the authors. “In contrast, the activation of T cells bearing high-affinity cetuximab-CAR was not affected by the density of EGFR.”

Dr. Cooper emphasized that the goal of the study was to make CAR-expressing T cells “differentiate friend from foe.” This approach, Dr. Cooper continued, “provides an advance in the field of CAR T-cell therapy because until now the focus in terms of T-cell activation was on the intracellular portion of the CAR design, which led to the development of second- and third-generation CARs with different abilities to signal T cells.

“Our study has shown that another possibility is to tweak the extracellular portion of the CAR that docks with the tumor by adjusting its affinity for the target protein. This technology can be used to develop CAR T cells that can be fine-tuned to target other overexpressed cancer proteins besides EGFR. An important derivative of this study is that scientists can now tweak, or modulate, the affinity of a CAR T cell to meet the needs of a given tumor.”